1. Field of the Invention
The present invention relates to a plug-in card for an optical transmission apparatus.
2. Description of the Related Art
As transmission information increases further in high-speed digital communication services, the speed and volume of transmission signals to be processed in the services also increase. For plug-in services for subscribers, a system employing an Ethernet (registered trademark) has been developed in addition to conventional digital dedicated lines offering a transfer rate of 1.5 megabits per second (Mbps) (DS1) or 45 Mbps (DS3). Hence, a variety of services are added to services provided by the SONET, which serves as a backbone network.
A multi-service provisioning platform (MSPP) is devised to incorporate a variety of services into a single platform. FIG. 31 is a schematic diagram of an optical transmission apparatus to which an MSPP is applied. As shown in FIG. 31, an MSPP apparatus 1 allows a selection of a plug-in card 2 to be mounted on the apparatus 1 according to the type of a required service. The MSPP apparatus 1 includes a control card 3 including a central processing unit (CPU), an on-use-side synchronous-transport-signal switch-fabric (STS-SF) card 4, a spare side STS-SF card 5, an on-use side synchronous (SYNC) card 6, and a spare side SYNC card 7.
To incorporate each service effectively into the MSPP apparatus 1, each plug-in card 2 (unit) has a path termination equipment (PTE) function, such as Ethernet over SONET (EoS) and virtual tributary (VT)-cross connect, and has a required function of terminating/generating a SONET STS path overhead byte (PTE function). FIG. 32 is a schematic diagram for illustrating incorporating the PTE function into an MSPP apparatus having conventional plug-in cards. In the specification and the drawings attached thereto, POH indicates path overhead, and a plug-in card for an optical transmission apparatus may be simply called plug-in card or card.
The MSPP apparatus shown in FIG. 32 includes a first plug-in card 8 having a POH generating circuit 9 and a POH terminating circuit 10, a first VT-SF card 11 having a POH generating circuit 12 and a POH terminating circuit 13, and a second VT-SF card 14 having a POH generating circuit 15. The POH generating circuits 9 and 12, and the POH terminating circuits 10, 13, and 15 are provided as circuits offering the PTE function. The second plug-in card 16 has no PTE function.
The plug-in cards 8 and 16 execute a SONET selection overhead (SOH)/line overhead (LOH) process and a PTE function process on input data. The processed data is put into an on-use side STS-SF card 4 and a spare side STS-SF card 5. Both STS-SF cards 4 and 5 execute circuit switching process on the input data signals. After the circuit switching process is over, the data signals are put out from the plug-in cards 8 and 16.
As necessary, the signals processed by the STS-SF cards 4 and 5 are put into the VT-SF cards 11 and 14 that make a selection on the signals from the on-use side and the spare side. The VT-SF card 11 and 14 execute a VT pointer process and a cross-connect process on the selected signals. The processed signals are then sent back to the STS-SF cards 4 and 5, and are put out of the plug-in cards 8 and 16. When the VT cross-connect process is unnecessary, the MSPP apparatus is not provided with the VT-SF cards.
In recent years, the volume of signals to be processed on a single plug-in card has been increasing with improvement of microfabrication technologies for semiconductors. This has raised a problem of a wider range of influence by a circuit error that happens when a plug-in card is switched from an on-use system onto a spare system for maintenance work. To avoid the problem, a demand is rising for completely interruption-free switchover function that is free from any error including one related to a path overhead. A system provided with such a function is thus expected to be developed.
FIG. 33 is a schematic diagram of a format of a SONET synchronous transport signal level 1 (STS-1) frame and the frame structure of a VT mapped in the payload of the SONET STS-1 frame. As shown in FIG. 33, the VT 23 of 28 channels is mapped in the payload 22 of the STS-1 frame 21. VT 23 is of a multi-frame structure including 4 frames, and a flag for identifying each frame is buried in an H4 byte 24 in the STS-1 path overhead of each frame. The H4 byte 24 represents an indicator.
A VT frame 25 is made up of 108 bytes (=27×4), where a V1 byte 26 and a V2 byte 27, which represent pointers, show the location of a V5 byte 29 that is the head of a VT payload 28. The STS-1 path overhead also includes a J1 byte 30 representing a path trace, and a B3 byte 31 representing a bit interleave parity (BIP)-8.
The following method is known as a method for carrying out interruption-free switchover between an on-use system transfer path and a spare system transfer path. According to the method, SDH virtual container (VC)-4 frames are transmitted in synchronization from an on-use system transmitting unit and a spare system transmitting unit to a receiving unit via an on-use system transfer path and a spare system transfer path, with ID signals multiplexed in the VC-4 overheads of the SDH VC-4 frames transferred through the on-use and spare system transfer paths. The SDH VC-4 frames from both on-use and spare systems are received by the receiving unit, where a phase difference between the VC-4 frames is detected, and a delay or precedence of the received VC-4 frame from one system against the same from the other system is judged based on the received ID signals from both systems. Based on a result of the judgment and the phase difference, the received data from the on-use system and the spare system are matched in timing and phase, and then switchover between the on-use system transfer path and the spare system transfer path is carried out (for example, Japanese Patent Laid-Open Publication No. 2000-196551).
Using conventional plug-in cards, however, may lead to a trouble. For example, when plug-in cards having the PTE function are mounted in redundant arrangement on an MSPP apparatus to execute the POH process and VT-pointer process, byte values, especially the values of J1 bytes and B3 bytes, in the path overheads sometimes become different between on-use side data and spare side data. A difference in the values of J1 bytes results for the following reason. FIG. 34 is a schematic diagram for explaining a difference between J1 byte values at the on-use side and the spare side. SONET GR-253 has a specified path trace function realized by using a J1 byte as consecutive 64 frames.
When a spare side card starts after the start of an on-use side card, or when path trace setup for the on-use side card and that for the spare side card are not carried out simultaneously, insertion timing for the head J1 byte becomes different between on-use data 41 and spare side data 42, as shown in FIG. 34. As a result, J1 byte output subsequent to the head J1 byte becomes different between the on-use side data 41 and the spare side data 42. When redundant switchover is carried out with J1 byte values remaining different, the 64-byte consecutiveness of J1 byte output from the SONET system is disrupted.
FIG. 35 is a schematic diagram of the disruption of consecutiveness of J1 bytes that happens when redundant switchover is carried out. As shown in FIG. 35, a J1 byte 43 as original output consists of consecutive 64 bytes. A J1 byte 44 output at the execution of redundant switchover, however, shows the disruption of 64-byte consecutiveness because the switchover causes the spare side data 42 to start in succession to the on-use side data 41. This causes an adjacent station (opposite node) monitoring a path trace to issue an alarm to inform the occurrence of path trace identifier miss match (TIM), which is an obstacle to network maintenance. The same thing happens in executing a path trace function (16 bytes/64 bytes) original to an SDH system.
A difference in the values of B3 bytes results with the following reason. FIG. 36 is a schematic diagram for explaining the reason for the difference. SONET GR-253 uses a specified B3 byte calculation method that requires a calculation result from a B3 calculation range in the previous frame to be included in every frame. Because of this, when a spare side card is started after the start of an on-use side card, or the order or time of card startup is shifted upon starting the SONET system to prevent simultaneous start of B3 byte calculation both at on-use side and spare side, subsequent calculation results from B3 calculation ranges, i.e., B3 byte values become different between the on-use side data 41 and the spare side data 42.
A difference between the on-use side and spare side in the values of a J1 byte or VT-pointer (V1, V2, V3, and stuff byte) included in a B3 calculation range also constitutes another reason for the difference between the B3 byte values at the on-use side and the spare side. An adjacent station (opposite node) calculates a B3 byte value from data sent from a transmission side unit, and compares a calculation result with a B3 byte value included in the next frame in the data.
FIG. 37 is a schematic diagram for illustrating comparison of B3 bytes executed at an adjacent station upon execution of redundant switchover. As shown in FIG. 37, the adjacent station receives the on-use side data 41 before the execution of redundant switchover, while receives the spare side data 42 after the execution of redundant switchover. When redundant switchover is carried out with B3 byte values remaining different between the on-use side and spare side, a calculation result [B3-A4] from a B3 calculation range in the on-use side data, which is given by the adjacent station just before the switchover, does not coincide with a B3 byte value [B3-S4] included in the spare side data 42 received by the adjacent station just after the switchover. In this case, the adjacent station detects a B3 error irrespective of whether subscriber data (main signal) is erroneous or correct, which hampers network maintenance work.